The objective of this project is to gain a better understanding of the molecular basis for leukemia and neurological disease development by utilizing retrovirus-based rodent model systems. By providing important information about the cause of disease, these animal models are invaluable for identifying and validating molecular targets relevant to human cancers for their prevention and treatment. [unreadable] Erythroleukemia induced in mice by the Friend spleen focus-forming virus (SFFV) provides an important model for understanding how deregulation of hematopoietic pathways can lead to leukemia. SFFV encodes a unique envelope glycoprotein, gp55,and its expression in the first stage of the disease allows erythroid cells to proliferate and differentiate in the absence of its normal regulator erythropoietin (Epo), causing erythroid hyperplasia and polycythemia. Our previous studies have shown that SFFV gp55 interacts with the Epo receptor complex and a short form of the receptor tyrosine kinase Stk, sf-Stk, at the cell surface to cause constitutive activation of sf-Stk and various components of the Epo signal transduction pathway necessary for proliferation, differentiation and survival of erythroid cells. Mice that do not express sf-Stk are resistant to SFFV-induced disease, indicating the necessity of this tyrosine kinase for mediating the biological effects of the virus. Studies are in progress to determine if blocking any of the pathways activated by SFFV through sf-Stk will inhibit the development of erythroleukemia. In the second stage of SFFV-induced erythroleukemia, which is analogous to blast crisis in human leukemia, virus-infected erythroid cells become blocked in differentiation due to expression of non-physiological levels of the myeloid transcription factor PU.1 and can be grown as immortal murine erythroleukemia (MEL) cell lines. Our studies demonstrate that unlike erythroid cells from the first stage of disease, these transformed erythroblasts have a block in Epo-induced STAT1 phosphorylation and DNA-binding and this correlates with high levels of the hematopoietic phosphatase SHP-1 in these cells, which may be dephosphorylating STAT1. When MEL cells are induced with chemicals to differentiate, PU.1 and SHP-1 levels go down and STAT1 DNA-binding increases, suggesting that STAT1 may be activating differentiation signals in these cells. Studies are in progress to determine if blocking SHP-1 or PU.1 expression in SFFV MEL cells will block their growth by inducing differentiation. We recently demonstrated that a pharmacological inhibitor of Jun-N-terminal kinase (JNK) blocks the proliferation and survival of SFFV MEL cells, suggesting that JNK may also be a valid molecular target for therapeutic intervention. Finally, we have shown that SFFV MEL cells metastasize to the bone marrow, where they are retained and cause meningeal leukemia, a common complication of human leukemia. By identifying molecular changes that allow SFFV MEL cells to engraft in the bone marrow, we hope to devise therapies to block this event. Although SFFV exclusively causes erythroleukemia in mice, our recent data suggests that this may be due to insufficient amounts of sf-Stk, the tyrosine kinase through which the virus mediates its biological effects, in cells outside of the erythroid lineage. We demonstrated that co-expression of SFFV gp55 and sf-Stk can transform rodent fibroblasts in the absence of the EpoR and that infection of mice with a bicistronic vector expressing SFFV gp55 and sf-Stk results in diseases not previously associated with SFFV infection, such as hemangiosarcomas. Our results suggest that Friend SFFV, in addition to causing erythroleukemia in mice, may also be capable of transforming other cell types that it infects provided that the cell expresses sf-Stk.[unreadable]